Could anyone tell me how to calculate the start up amps of my vacuum pump, The information I have is:

Thomas motor. 115v 60HZ 4.2amps 370V AC cap rat 15mfd AMB 40c

Any help would be much appreciated.

  • Hello, and welcome to Home Improvement. Unfortunately, I doubt you'll get a good answer here. Please take our tour to see how better to participate in our forum. – Daniel Griscom Jan 12 at 3:11
  • It would help if you edited your question and tell us why you need to know this information. NEC 430 dedicates an entire section on motors and it's quite extensive, so we would need to know so we can narrow it down to an answer. FYI - 4'th year apprenticeship training can dedicate up to six months on NEC section 430. – Retired Master Electrician Jan 12 at 14:13

This is how to calculate it, but you are going to need more information.

I don't know your particular motor, but it is very high probability it is an asynchronous, single phase type with a single capacitor and starter coil. If that's wrong, please let me know and I'll change my answer.

Edit: Disconnect it from the mains or any power supply before attempting any of the procedure below. Always worth adding a safety warning, just in case.

  • We would need to know the DC resistance of the motor for which you would need to short circuit the capacitor using a clip lead or getting a mate to manually short it with wire (being aware of any residual charge in the capacitor that could present danger), then place the motor across an ohmmeter ie, measure across the live and neutral at the plug (you'll find an ohmmeter on any £10 or $15 multimeter from a hardware store). The reading will be in ohms and should be single or low double digits for your motor.

  • You can reasonably ignore the equivalent series resistance of the capacitor as it's negligible in this case.

The maths is: Vpeak = IR, where Vpeak = √2(V) - ie 1.414 multiplied by the RMS value of the mains voltage where you are that you're running it on.

  • So, say for a mains voltage of 240v (RMS) and a DC resistance of 20 ohms you would calculate:

240*1.414 = 339.36 (near enough 340) you would then divide this by the resistance.

340/20 = 17Amps surge current.

  • Or for a supply voltage of 110 volts and the same resistance of 20 ohms:

110*1.414/20 = 7.8 Amps surge.

  • A typical result for your motor would be in the order of 6 times it's on-load current consumption - in your case 4.2*6 = 25 Amps surge, but to be sure, you'd need to measure as above.

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